Coronary artery disease (CAD) is the leading cause of death among men and women in the United States, accounting for approximately one of every five deaths. Approximately 2.4 million adults have a history of myocardial infarction (MI), angina, or both, and it is estimated that millions of others have clinically silent coronary artery disease.
The traditional view has been that myocardial ischemia in CAD results from atherosclerotic plaques that narrow the vessel's lumen and decrease myocardial blood supply. However, research has shown that the reduction of blood flow results from the combination of fixed vessel narrowing and abnormal vascular tone, contributed to by atherosclerosis induced endothelial cell dysfunction.
Atherosclerotic coronary arteries can be treated by several methods. These treatment methods are divided into two major categories: non-invasive and invasive. Non-invasive treatments of coronary artery disease involve both medication and reduction of controllable risk factors of atherosclerosis. Non-invasive treatments cannot improve the coronary circulation when symptoms associated with coronary heart disease are severe. In such cases, invasive treatments are required to improve blood flow to the heart muscle. The most common invasive treatments for coronary heart disease include coronary artery bypass surgery (CABG), Percutaneous Transluminal Coronary Angioplasty (PTCA) and the use of percutaneously introduced prosthetic devices, namely, coronary stents. In CABG, a superfluous vein is removed from the patient's leg and sewn into the blood vessels that supply oxygenated blood to the heart. This transplanted vein, known as a graft, carries blood to the heart muscle, bypassing atherosclerotic areas in the coronary arteries. Alternatively, an internal mammary artery can be directly anastomosed distal to the stenotic site. CABG has become the most common major operation in the United States. It is usually a safe procedure with a ten-year patency rate of approximately 80–90 percent. Coronary bypass surgery effectively relieves chest pain (angina pectoris), increases exercise capacity, improves heart function in some patients, and prolongs life in certain patients. With current techniques, CABG requires a one-week hospitalization.
In PTCA, the lumen of an atherosclerotic coronary artery is increased by the inflation of an intravascular balloon catheter. Today, balloon angioplasty is used successfully to treat atherosclerosis in both systemic (peripheral, renal, cerebral) and coronary arteries in selected patients. The success rate of PTCA by an experienced staff ranges from 80 to 95 percent, and the average hospital stay after PTCA is roughly two days. Advantages of PTCA include decreased hospital stay, lower cost, decreased recuperation time, and no requirement of general anesthesia or chest incision. Although these advantages contributed to PTCA becoming a widely used alternative to coronary artery bypass surgery, the underlying mechanisms of balloon angioplasty continue to be elucidated. PTCA was originally thought to increase the lumen by compacting the atheromatous material within a relatively unyielding artery. Histological studies indicate, however, that the mechanisms of balloon angioplasty are much more complex and that the two primary mechanisms of this procedure are: (a) “over-stretching” of the vascular wall, which causes some mural injury, and (b) plaque fracture with partial separation from the vascular wall. This localized damage of the arterial wall is essential for a successful dilation via balloon angioplasty. Nevertheless, it is assumed that excessive damage likely contributes to complications such as restenosis, dissection, vasospasm, and rupture. Although this vascular wall damage results from mechanical forces, it is not known at which level of mechanical stress or strain it occurs.
The limitations of PTCA cited above resulted in the development of new technologies including atherectomy, laser angioplasty and coronary stenting.
The most significant change in interventional cardiology has been the growth of coronary stents, endovascular scaffolding devices that maintain the luminal integrity of diseased blood vessels. A typical stent implantation involves pre-dilatation of the target lesion via a PTCA procedure followed by implantation of the coronary stent in the same area of the coronary artery.
Clinical studies have shown that coronary stents improve the safety of PTCA by successfully treating abrupt and threatened closure, improve long-term clinical outcomes by reducing restenosis, and provide predictable angiographic results. Currently, coronary stents are available in several basic configurations: slotted tube, coil, mesh, ring, and multi-design. All are made of metal. Most stents are made of a plastically deformable material such as 316L stainless steel. Delivery of plastically deformable stents is commonly accomplished by the use of a balloon catheter. The balloon expandable stents are generally made of plastically deformable materials such as 316L stainless steel, elgiloy, nitinol, platinum-iridium and tantalum. They exhibit initial recoil due to elastic/plastic behavior of the plastically deformable stent material and initial vascular constriction. Once deployed, they provide fairly rigid scaffolding to the arterial lumen, thereby preventing abrupt closure and dissections. Self-expanding stents are generally secured on a delivery catheter under a constraining retractable sheath. Elgiloy and nitinol are the materials used for most self-expanding stent designs. Unlike the balloon expandable stents, which have some degree of recoil, self-expanding stents tend to continue expanding to their fixed diameters after deployment. Self-expanding stents do not expand beyond their pre-set nominal size. Therefore, pre-procedure proper sizing of the self-expanding stent is very important to prevent extensive vessel injury. It is not uncommon that both self-expanding and balloon expandable stents are post dilated to achieve optimum stent opposition. There are other stent designs such as bio-absorbable stents that are commonly manufactured using polymeric materials. They too are deployed using a balloon catheter. The primary advantages of such a stent over a metallic stent are: (a) there is a better match between stent stiffness and that of the vessel wall; and (b) they are designed to bio-absorb in the body after the diseased and injured vessel wall is healed. The disadvantages of such devices include low radial stiffness and visibility, and difficulty in identifying a biocompatible polymer for use in coronary arteries.
Clinically desirable stent characteristics include: minimal recoil; axial flexibility; low crossing profile; minimal surface area; adequate radial strength; good fluoroscopic visibility and good thrombo-resistance. Currently, none of the available stent designs incorporates all of these characteristics. Each stent design has its specific advantages and disadvantages. Acute clinical complications associated with coronary stenting include thrombosis (sub-acute), restenosis, bleeding due to required anticoagulation regimens, embolization, side-branch occlusion, coronary perforation and delivery balloon rupture.
Although several clinical studies have demonstrated that coronary stenting (when compared with PTCA alone) reduces restenosis rates in selected atherosclerotic lesions, restenosis within the stent (in-stent restenosis) remains a clinical problem. The incidence of in-stent restenosis has increased with recent advances in stent designs allowing stenting in difficult or less-than-ideal lesions, which leads to an intrinsically higher likelihood of restenosis following percutaneous intervention. It is estimated that of the 725,000 percutaneous coronary interventions completed in the United States, more than 80% will receive a new stent, and that 100,000 of these cases will develop symptoms due to in-stent restenosis.
The arteries, whose primary function is to carry blood from the heart to the capillaries, are generally subdivided into two categories. Those with large diameters and with many elastic constituents are called elastic arteries (e.g., aorta, carotids), whereas smaller diameter vessels are categorized as muscular arteries (e.g., coronaries, femorals). All arteries have similar wall organization and consist of three concentric layers.
The intima, or innermost layer of an artery, mainly consists of endothelial cells that line the vascular wall, the basement membrane, and a subendothelial layer. The subendothelial layer usually is present in large arteries with the exception of some special types of muscular arteries such as coronaries. The endothelial cells of the coronary arteries are elongated along the longitudinal axis of the vessel and constitute lineal folds. These endothelial folds of coronary arteries contribute to smooth peripheral blood flow and prevent the adherence of elements to the endothelial surface. The media, or middle layer of the artery, is composed of smooth muscle cells, a varied number of elastic sheets (laminae), bundles of collagen fibrils and a network of elastic fibrils all of which are embedded in a viscous gel matrix. The structure of the media is significantly different in elastic and muscular arteries. In elastic arteries, fenestrated elastic laminae, averaging 3 mm in thickness, are concentrically arranged and spaced equidistantly. Their numbers depend on the size of the elastic artery and vary between 20 and 60. A network of delicate elastic fibrils interconnects these elastic laminae. The smooth muscle cells are located within this framework. The majority of the smooth muscle cells are oriented obliquely, and run diagonally at small angles. The elastic components and the smooth muscle cells are held together by a network of collagen fibrils with two distinct and separate modes of fiber organization. The media of muscular arteries are distinguished by distinct internal elastic laminae separating the intima and the media, and less distinct external elastic laminae marking the outer border of the media. Compared to elastic arteries, muscular arteries do not have intermediate layers of concentric elastic laminae except in larger muscular vessels. Instead, muscular arteries consist of concentrically arranged populations of spindle-shaped smooth muscle cell layers. Concentrically arranged smooth muscle cells form helices that are invested within thin external lamina and narrow layers of delicate collagenous fibers. There is also a very fine network of elastin fibers throughout the media of the muscular arteries.
The adventitia, or the outermost layer of the arterial wall, consists primarily of longitudinally and circumferentially oriented collagen bundles, connective tissue cells, vasa vasorum, and a loose network of randomly oriented thin elastic fibers. Nerve cells and fibroblasts are also present in the adventitia. The thickness of the adventitia is not constant around the perimeter of the coronary arteries.
Arteries have been found to be thick-walled, nearly cylindrical, heterogeneous, anisotropic and nonlinearly viscoelastic composite materials. They undergo large deformations, relax when held at a constant strain, creep when subjected to a constant stress, and exhibit hysteresis when subjected to cyclic loading and unloading. Following preconditioning (i.e., sufficient number of loading cycles), their stress-strain relationship becomes unique, repeatable and predictable. In recent years in vivo studies performed on the coronary arteries to evaluate the role of coronary artery smooth muscle in the control of coronary circulation. These studies show not only a large potential contribution of coronary artery smooth muscle to arterial properties, but also the presence of a significant tone in coronary smooth muscle. The relative significance of this smooth muscle in the production of coronary spasm and in contributing to acute angina, however, remains unclear.
Biological tissues respond to mechanical stresses by growth and resorption when these stresses are applied over a long period of time. When the heart is overloaded its muscle cells increase in size. For example, the volume-overloaded heart increases its ventricular volume while a pressure-overloaded heart increases its wall thickness. When the oxygen supply to the lung is reduced suddenly, pulmonary blood pressure increases. As a result, the thickness of the adventitia and the smooth muscle layer in the pulmonary arteries increase, the lumens of small arteries decrease, and new muscle growth occurs on smaller and more peripheral arteries than normal. Pathological differences are observed (e.g., neointimal hyperplasia) in arteries stented via balloon-expandable versus self-expanding stents where slow growth of self-expanding stents yields greater late gains than those associated with the immediate large lumens achieved with balloon-expandable stents.
When shear stress due to blood flow artificially increases, histological and biochemical changes occur in the aortic endothelium. The transport of matter through cell membranes by active or passive mechanisms depends on strain in the cell membranes. Chemical reaction rate depends on pressure, stress, and strain. Therefore, it is reasonable to conclude that the remodeling of soft tissues involving growth and resorption of cells and extracellular materials is linked to the stress and strain in the tissue.
The determination of the failure characteristics of living tissues and organs is complex. There are many ways a material can fail in a biological sense. To study the failure characteristics of biological materials there must be correlative clinical observations and pathological examinations with stress and strain in the tissues. For example, soft tissues have very different stress-strain relationships than those of the typical engineering materials. Hysteresis, creep, and relaxation exist in soft tissues, and the strength of most soft tissues depends on the strain rate. In the physiological range, the stress in the soft tissue generally increases exponentially with increasing strain. At a certain strain higher than physiological, the tissue yields and breaks. Acute morphologic changes due to PTCA include endothelial denudation extrusion of fluid from the lesion, fracture of the plaque with partial separation from the underlying media, and “over-stretching” of the vascular wall. The majority of the damage occurs in the intima and the inner half of the media, medial damage consists primarily of smooth muscle cell layer disruption including torn cell-to-cell and cell-to-elastin connections, the intimal and medial damage increases with increasing dilation, and the adventitia remains largely undamaged. Any mechanical trauma, which alters a tissue morphologically, also produces physiologic changes. In the case of balloon angioplasty, long term effects include platelet and fibrin deposition on the damaged endothelium, formation of neointima, removal of damaged myocytes by macrophages, edema, and synthesis of new collagen by fibroblasts. Physiologic changes also include differences in sodium pump activity, prostacyclin release, and changes in the artery's nutrient supply and metabolism. In summary, the general characteristics of arterial damage due to PTCA can be summarized as follows: intimal and medial damage, increasing with dilation; intramural damage greater in the inner wall than in the outer wall; longitudinal intimal-medial dissection; medial damage consisting primarily of disrupted smooth muscle, including torn cell-to-cell and cell-to-elastin connections and torn elastic (some collagen) fibers.
These findings are consistent with known stress distributions in non-uniformly inflated cylinders. That is, in addition to higher stress concentrations in the inner walls, shearing stresses are produced in the walls of the cylinder in the tangential-axial (EZ) and radial-axial (RZ) planes. Data on uniformly hyper-distended vessels further suggest that the smooth muscles (and basal lamina) probably tear first. Muscle tears more easily than connective tissue and is probably why muscle is often surrounded by protective connective tissue sheaths. Elastin appears to tear before collagen and is consistent with mechanical data. The modulus of elastin is orders of magnitude less than that of collagen, and elastin is less undulated than collagen. Thus elastin becomes taut well before the collagen. Because of this, it is a common misconception that a balloon exerts only a radial pressure on the intimal surface. Since vessels usually experience only normal stresses (except near branch sites), they are not designed to resist this type of load and may therefore be susceptible to shear induced tearing.
The primary obstacle to the long-term success of PTCA and coronary stenting is restenosis. The restenosis occurs within three to six months in 25–50% of patients who have PTCA and/or stenting procedures. Costly additional interventional or surgical procedures are usually required to treat the restenosis. It is well accepted that vascular restenosis or renarrowing of the lumen of the coronary artery is a direct response of the vessel injury incurred during revascularization. In a recent paper, it was stated that the common denominator for all current revascularization techniques appears to be vessel wall injury, independent of revascularization method (i.e., removal of vascular endothelium and exposure to deep tissue components).
Utilizing an animal model, Robert Schwartz showed that restenosis and the related neointimal response have a direct correlation to the amount of coronary artery injury, regardless of the intervention type. This observation is consistent with the restenosis rates reported in the literature for small vessel interventions. Although the same sizing criterion (balloon to reference vessel diameter) is used, the circumferential stretch ratio in the wall of small vessel would be higher than that of the larger diameter vessel, and thus, the amount of vascular injury in the small diameter vessel would be much higher. Similarly, the treatment of a long, diffused lesion with PTCA would require inconsistent “over-stretching” between the normal and diseased segments of the target vessel and thus result in increased vascular damage. There are several mechanisms that lead to restenosis: thrombosis; inflammation; smooth muscle cell migration/proliferation; and extra-cellular matrix formation/degradation. These represent the fundamental sequence of response to injury. The final clinical consequence of these mechanisms is late lumen renarrowing due to neointimal hyperplasia.
Historically, restenosis has been treated with medical therapy, balloon angioplasty, or CABG surgery. Specifically, if the original invasive procedure was angioplasty without stenting, the next step, once restenosis occurs, is usually to repeat the angioplasty followed by stent implantation. If restenosis occurs within a stent, it is usually treated by repeat angioplasty. However, once restenosis has occurred, the chance of a second restenosis nearly doubles.
Over the past decade, several new devices and strategies have been developed to reduce restenosis. These include directional, rotational and extractional atherectomy devices, excimer laser angioplasty and cutting balloon, but no therapy has consistently achieved reduction in restenosis. The focus of the prevention of restenosis has also been on the use of pharmacological agents. Local drug delivery devices, including drug-eluding stents, and even more sophisticated cell-based vascular gene-delivered systems have been developed. The clinical application and efficacy of such therapies, however, remains to be demonstrated.
Finally, the United States Food and Drug Administration (FDA) recently approved a new therapy, two catheter-based radiation therapies for the treatment of in-stent restenosis. However, the long-term safety of this novel therapeutic approach has been argued. There are associated problems such as edge restenosis, late thrombotic occlusions and potential delayed restenosis. In addition, incorporating intracoronary radiation in daily clinical practice has practical problems.
The nature of both PTCA and/or stenting (with pre-dilatation) techniques is that they induce some degree of injury on the coronary artery wall via balloon dilatation. It is generally accepted that this so-called “controlled” wall injury is required to achieve the desired acute and long-term clinical outcomes. Inherent in response to this injury, restenosis remains to be a major clinical problem. Although, limiting the amount of injury during revascularization has been proposed as a means to limit the restenosis, there has been no attempt made to develop this idea further.
It would be desirable, therefore, to provide an interventional method that limits the amount of injury during revascularization and that comprises an alternative method to PTCA and current stenting practice. Such a revascularization technique would minimize vascular injury, and therefore, the restenosis rates associated with PTCA and stenting. The vascular wall would not be injured (or the injury would be minimal) when it is stretched at an actively controlled manner (or strain rate) where stresses in the vascular wall remain at or below the physiologic levels at all times. Under such loading conditions, the vascular tissue including the endothelial layer would not be exposed to a major mechanical trauma. Instead, the vascular tissue would adapt (condition) or positively remodel in response to this actively controlled stretching. The remodeling would be different than the remodeling associated with the restenosis process (i.e., vascular response to injury—neointimal hyperplasia following PTCA). This device would provide adequate initial scaffolding upon deployment and allow actively controlled expansion (gradual or step-wise) of the vascular wall. The novelty of this method is that adequate flow through the diseased segments of blood vessels (or conduits) would be achieved by an actively controlled stent-like implantable device. Preferably, the device would be motorized (e.g., by Micro-Electro-Mechanical Systems, MEMS) or otherwise mechanically actuated to provide controlled, gradual or step-wise expansion of blood vessels. The controlled, gradual or step-wise expansion would prevent trauma to the tissue, thus eliminating potential long-term complications (such as restenosis), and allow blood vessels (the living tissue) to remodel in a controlled manner. The utility of this method is that, with MEMS and/or nano technology, it is possible to design and build such a state of the art interventional device.